The most common type of asexual reproduction is apogamy, whereby a sporophyte plant develops from a gametophyte without fertilization, similar to apomixis in angiosperms. In naturally occurring apogamous species, the viable spores produced by the sporophyte have the same chromosome number as the sporophyte Walker, , Obligate apogamy often occurs naturally in species of ferns that produce no or only one type of gametangia.
Because water is required for the flagellated sperm to swim to the egg in ferns, apogamous species are typically found in dry habitats where water is limiting White, Apogamy also can be artificially induced in many ferns by adding sucrose to the culture media in which gametophytes are grown Whittier and Steeves, ; White, By optimizing the conditions for inducing apospory in Ceratopteris richardii gametophytes, a recent study has established C.
Induced apogamous sporophytes of C. To better understand how sucrose promotes the development of a sporophyte from cells of the gametophyte, the same researchers identified genes whose expression is up-regulated during the period of apogamy commitment.
Many of them are associated with stress and metabolism or are homologs of genes preferentially expressed in seed and flower tissues Cordle et al. Understanding apogamy, coupled with studies of apospory in C. A second form of asexual reproduction in homosporous ferns involves vegetative propagation of the gametophyte. While relatively rare, such gametophytes typically do not produce sex organs. The fern Vittaria appalachiana , for example, is only known from its gametophytes Farrar and Mickel, Each gametophyte forms vegetative buds, or gammae, that allow gametophytes to multiply and form mats in dark, moist cavities and rock shelters in the Appalachian Mountains.
While the origin of V. Most homosporous ferns that reproduce sexually ultimately form hermaphroditic gametophytes that have antheridia and archegonia. While hermaphroditism increases the probability that a single gametophyte will reproduce, self-fertilization of a hermaphrodite which is genetically similar to a doubled haploid in angiosperms results in a completely homozygous sporophyte.
Given that this absolute inbreeding could have negative consequences to the individual and reduce genetic variation in populations, it is not surprising that homosporous ferns have evolved mechanisms to promote outcrossing. One such mechanism that is common to many species of ferns involves the pheromonal regulation of sexual identity, where the sexual phenotype of an individual gametophyte depends on its social environment.
Antheridiogens or antheridiogen responses have since been identified in over 20 species of ferns Yamane, ; Kurumatani et al. This response is illustrated here for the fern C. The hermaphrodite also secretes antheridiogen, or A CE for antheridiogen Ceratopteris into its surroundings. In a population of spores, spores that germinate first become hermaphrodites that secrete A CE , while slower-growing members of the population become male in response to the secreted A CE.
In comparison to chromosomal based sex determination, this mechanism of sex-determination is unusual because it allows the ratio of males to hermaphrodites to vary depending on population size and density and it is inherently flexible rather than fixed. The antheridiogen response in C. A single spore always develops as a hermaphrodite when grown in the absence of A CE.
The hermaphrodite consists of a single sheet of cells with a distinct multicellular meristem that forms a meristem notch and multiple archegonia that develop adjacent to the meristem notch, which are highlighted in the SEM boxed area of the hermaphrodite.
The male lacks a meristem and almost all cells differentiate as antheridia. The SEM shows six antheridia, each having a ring cell and a cap cell that pops open to release sperm. When a male gametophyte is transferred to media lacking A CE , some cells divide and begin to form a hermaphroditic prothallus.
Typical of other ferns, a C. The lateral meristem not only confers indeterminate growth to the gametophyte, but its formation coincides with a loss in ability to respond to A CE as well as the secretion of A CE.
Archegonia invariably initiate close to the meristem notch of the hermaphrodite, well after the lateral meristem is well developed. While the hermaphroditic program of expression cannot be reversed, the male program of expression is reversible. Antheridiogen thus serves multiple functions in male gametophyte development: it represses divisions of the prothallus that establish the lateral meristem; it promotes the rapid differentiation of antheridia; it represses its own biosynthesis; and it serves to maintain in the gametophyte an ability to respond to itself.
All of the antheridiogens that have been structurally characterized from ferns are gibberellins GAs Yamane et al. Although the structure of ACE is unknown, GA biosynthetic inhibitors reduce the proportion of males in a population of C.
Most recent studies aimed at understanding how antheridiogen determines the sex of the gametophyte have focused on two species of homosporous ferns: C. Ceratopteris richardii is a semi-tropical, annual species and is useful as a genetic system for many reasons. Large numbers of single-celled, haploid spores typically 10 6 can be mutagenized and mutants identified within 2 weeks after mutagenesis. Gametophytes can be dissected and regrown, making it possible to simultaneously self-fertilize and out-cross a single mutant gametophyte.
Because C. Over 70 mutants affecting sex determination have been characterized, most falling into three major phenotypic groups: the hermaphroditic her mutants, which are hermaphroditic in the presence or absence of A CE , the transformer tra mutants, which are male in the presence or absence of A CE , and the feminization fem mutants, which are female in the presence or absence of A CE and produce no antheridia.
Through test of epistasis i. This pathway reveals that there are two major regulators of sex: TRA , which is necessary for lateral meristem and archegonia development female traits , and FEM , which is necessary for antheridia development the male trait.
TRA promotes the development of a gametophyte with female traits and represses the development of antheridia by repressing the FEM gene that promotes male development. What is remarkable about this pathway is that it is inherently flexible, which is consistent with what is understood about sex determination in this species by A CE.
The SD pathway in C. T bars represent repressive events whereas arrows indicate activating events. While this model explains how male and female gametophyte identities are determined, it does not explain the hermaphrodite. One possibility is that in certain cells of the hermaphrodite, the activities of FEM and TRA are reversed, allowing FEM to be expressed in cells that will eventually differentiate as antheridia. Testing this and other possibilities will require the cloning of the sex-determining genes and assessing their temporal and spatial patterns of expression in the developing hermaphrodite.
The sex-determining pathway in C. Based on the similarities between the GA signaling pathway in angiosperms and the sex determination pathway in C. Lygodium japonicum is another homosporous fern species with an antheridiogen response. This species has the distinct advantage of having its antheridiogens structurally well characterized. Two different GAs have been identified as antheridiogens in this species, including GA 9 methyl ester Yamane et al. If the sperm do manage to get to an egg, fertilisation occurs, and that is where the two — the sperm and egg — come together.
That doubles the number of chromosomes and that gives rise to a whole new typical fern plant again, and the cycle repeats. The zygote develops from the prothallus fern gametophyte. It grows using mitosis and develops into a young fern plant.
The mature fern plant consists of three major parts — the rhizome, the fronds and the sporangia. The mature fern plant is the sporophyte structure that produces spores, which are released from sporangia. Sori are clumps of sporangia that hold the reproductive spores.
They are found on the underside of fern fronds. The sporangia is the structure which produces spores. In ferns, the sporangia are usually aggregated into particular bigger structures. If you turn over a fern frond and you might see lines, and those are aggregations of the sporangia. Not every plant grows from a seed. Some plants, like ferns and mosses, grow from spores.
Other plants use asexual vegetative reproduction and grow new plants from rhizomes or tubers. We can also Ferns are green flowerless plants with divided leaves that tend to grow in damp, shady areas.
The developing leaves of Add to collection. Transcript Meiosis Meiosis is a type of cell division that produces gametes — cells that contain half the number of chromosomes than the parent cell. Transcript Dr Leon Perrie The typical big fern plant, what it does is, by meiosis, produces spores, and the spores have half the number of chromosomes of the big parent plant.
Transcript Dr Leon Perrie So a spore is the product of meiosis. The pollen tube develops slowly as the generative cell in the pollen grain divides into two haploid sperm cells by mitosis. At fertilization, one of the sperm cells will finally unite its haploid nucleus with the haploid nucleus of an egg cell.
Female cones ovulate cones contain two ovules per scale. One megaspore mother cell megasporocyte undergoes meiosis in each ovule. Three of the four cells break down leaving only a single surviving cell which will develop into a female multicellular gametophyte.
It encloses archegonia an archegonium is a reproductive organ that contains a single large egg. Upon fertilization, the diploid egg will give rise to the embryo, which is enclosed in a seed coat of tissue from the parent plant. Fertilization and seed development is a long process in pine trees: it may take up to two years after pollination.
The seed that is formed contains three generations of tissues: the seed coat that originates from the sporophyte tissue, the gametophyte that will provide nutrients, and the embryo itself. In the life cycle of a conifer, the sporophyte 2n phase is the longest phase.
The gametophytes 1n , microspores and megaspores, are reduced in size. This phase may take more than one year between pollination and fertilization while the pollen tube grows towards the megasporocyte 2n , which undergoes meiosis into megaspores. The megaspores will mature into eggs 1n.
Life cycle of a conifer : This image shows the life cycle of a conifer. Pollen from male cones moves up into upper branches where it fertilizes female cones.
Gymnosperms are a diverse group of plants the protect their seeds with cones and do not produce flowers or fruits. Modern gymnosperms are classified into four phyla. The first three the Coniferophyta, Cycadophyta, and Gingkophyta are similar in their production of secondary cambium cells that generate the vascular system of the trunk or stem and are partially specialized for water transportation and their pattern of seed development.
However, these three phyla are not closely related phylogenetically to each other. The fourth phylum the Gnetophyta are considered the closest group to angiosperms because they produce true xylem tissue. Conifers are the dominant phylum of gymnosperms, with the most variety of species. They are typically tall trees that usually bear scale-like or needle-like leaves.
Water evaporation from leaves is reduced by their thin shape and the thick cuticle. Snow slides easily off needle-shaped leaves, keeping the load light and decreasing breaking of branches. Adaptations to cold and dry weather explain the predominance of conifers at high altitudes and in cold climates.
Conifers include familiar evergreen trees such as pines, spruces, firs, cedars, sequoias, and yews. A few species are deciduous, losing their leaves in fall. The European larch and the tamarack are examples of deciduous conifers. Many coniferous trees are harvested for paper pulp and timber. Diversity of conifers : Conifers are the dominant form of vegetation in cold or arid environments and at high altitudes.
Shown here are the a evergreen spruce Picea sp. Notice the yellow leaves of the tamarack. Cycads thrive in mild climates. They are often mistaken for palms because of the shape of their large, compound leaves. Cycads bear large cones and may be pollinated by beetles rather than wind, which is unusual for a gymnosperm.
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